The present invention relates to a microwave amplifier and, more particularly, to a microwave amplifier for amplifying a microwave signal including a plurality of different carrier frequencies.
In general, in a satellite communication system and the like, communication is performed by using a microwave (ultra-high frequency wave) modulated by an input signal having a comparatively low frequency. The transmitter of such a communication system employs a microwave amplifier using an active element, e.g., an FET (Field Effect Transistor), in order to amplify the microwave signal with a high gain.
Conventional, in such a microwave amplifier, a DC bias voltage is supplied through a .lambda.g/4 line (where .lambda.g is the wavelength on the microstrip line), having a high impedance with respect to the carrier frequency of the microwave signal, as a means for supplying an appropriate DC bias voltage to the active element.
FIGS. 9A and 9B show the circuit diagram and the mounted state, respectively, of the main part of a conventional microwave amplifier. Referring to FIG. 9A, one end of a .lambda.g/4 line 111 is connected to a drain D of an FET 110 a source S of which is grounded. The other end of the .lambda.g/4 line 111 is connected to a DC bias voltage V.sub.DS and grounded through a capacitor 115 that short-circuits an input signal. These elements are mounted with the layout shown in FIG. 9B.
While a high impedance is maintained with respect to the carrier frequency and the input signal frequency is short-circuited by a DC bias voltage supply circuit constituted by the .lambda.g/4 line 111 and capacitor 115, the DC bias voltage V.sub.DS is supplied to the drain D of the FET 110. This circuit arrangement is described in, e.g., Japanese Patent Laid-Open No. 61-35006.
In this conventional microwave amplifier, since a pure resistance component R of the .lambda.g/4 line 111 for supplying the DC bias voltage V.sub.DS is very low, the voltage drop caused by the pure resistance component R can be decreased. However, a reactance component jX of the .lambda.g/4 line 111 is not considered. Accordingly, the low-frequency beat signal generated in the presence of a large number of carrier frequencies as the carrier frequencies of the microwave signal decreases the DC bias voltage V.sub.DS with the reactance component jX of the DC bias voltage supply circuit in this beat frequency band.
For example, when two carrier signals having different carrier frequencies f1 and f2 (f1&lt;f2) are mixed, a beat signal having as its frequency the frequency difference f2-f1 of these two frequencies is generated. As a result, the DC bias current supplied to the drain D of the FET 110 also changes in accordance with the beat signal.
The .lambda.g/4 line 111 through which the DC bias current flows has an impedance of R+jX. The drain voltage V.sub.DS (t) of the FET 10 is represented by: EQU V.sub.DS (t)=V.sub.DS -I.sub.D (t).multidot.(R+jX)
where I.sub.D (t) is the current of the frequency component of the signal.
When the beat signal described above is generated, the drain voltage V.sub.DS (t) of the FET 110 is fluctuated by the current I.sub.D (t) of the frequency component, the pure resistance component R, and the reactance component jX. It has become apparent that this interferes with the bias voltage to fluctuate the DC bias voltage of the drain D, thereby causing distortion in the amplified output. In other words, the present inventors have found that the beat signal is the cause of distortion in the output.